During an unloading operation, the rupture disk on a tank car used as temporary storage and loaded with concentrated oleum burst at 3.8 bar even though it was calibrated at 7 bar. Upon hydrolysing in contact with the air, the SO3 formed 13 km-wide and 300-m thick cloud of toxic H2SO4 aerosol that remained over the sector for 4 hours.

Schools were evacuated. The population was confined indoors and traffic was stopped. An estimated 8,000 people were effected by the respiratory and skin problems after having inhaled the cloud’s toxic fumes. Of these people, 6 284 were admitted for examination at local hospitals. Only 7 people were hospitalised more than 24 hours. A worker managed to plug the leak 4 hours after it started.